Mesoplodon traversii (Gray, 1874) is shown to be a senior synonym of the recently described beaked whale Mesoplodon hahamondi Reyes et al., 1995 on the basis of a phylogenetic analysis of mitochondrial DNA control region sequences. The mandible and teeth of M. traversii, first reported in 1873 by Hector as Dolichodon layardii. are redescribed. The species can be distinguished by features of the calvaria; including the large jugal, broad rostrum, and small distance between premaxillary foramina. The male teeth, which are large and spade-shaped with a strong terminal denticle, are also diagnostic. M. traversii is known only from Pitt Island and White Island, New Zealand and Robinson Crusoe Island, Juan Fernandez Archipelago, Chile.